Swirling Airflow Through a Nozzle: Choking Criteria

Since the mass flow rate through nozzle is primarily a function of throat density and axial Mach number, the reductioninthelatterwithswirlexplainstheobservedreductioninmass flowatmatchedreservoirpressure.Greater pressures, on the other hand, result in higher throat densities, which compensates for the reduced axial Mach number,andthemass flowratecanbekeptconstantatitsnonswirlingvalue.Itwasalsofoundthatthedistributionof subsonicMachnumber(andnotanyofitscomponents)inaswirling flowissolelydependentoncross-sectionalarea, similar to nonswirling flows; i.e., nonswirling and swirling flows have the same subsonic Mach number profile. In terms of thrust and specific impulse, the application of swirl at matched nozzle reservoir pressure results in the expected reductions in discharge coefficient, thrust, and specific impulse. At matched mass flow, however, the application of swirl results in the enhancement of both thrust and specific impulse. This is attributed to the considerable degree of underexpansion associated with the swirling flow as a result of the higher nozzle reservoir pressure with swirl.